[Proj] Using Proj.4 to compress a 10-parameter Molodensky-Badekas datum shift into 7 parameters

[Mikael Rittri]

Clifford,
thanks for your comments.

You wrote:
> In regard to the "unexplained" difference in ellipsoid height results,
> my guess is the approximations made by the inverse algorithm 
> from Geocentric coordinates.

Well, I hope it is just something like that.  But I don't know how 
accurately cs2cs implements this inverse algorithm.

> The "evaluation point" is the Datum Origin, a rather important item.

"Evaluation point" is the term used in the EPSG database, but I see
that they sometimes call it "rotation point" in their Guidance Note 7.2.

No, it isn't.

I mean, the datum origin may be important, but the origin of the
La Canoa datum is at
   
63° 51' 34.880" W, 08° 34' 17.170" N
(source: your column on Venezuela, http://www.asprs.org/resources/grids/12-2000-venezuela.pdf )

And when I convert the evaluation point of my example datum shift
to long/lat, I get a different location:

>cs2cs +ellps=intl +proj=geocent +to +ellps=intl +proj=longlat
2464351.59   -5783466.61   974809.81
66d55'15.629"W  8d52'22.857"N -16138.645

which is about 340 km west of the datum origin, and nearer
the centre of Venezuela.

Rather, I think the evaluation point in this example
might be the average of all observation points used
to derive the datum shift.  Noel used this approach in
pages 45 - ... of http://www.hydrometronics.com/downloads/Molodensky-Badekas.pdf
That could also explain why the evaluation point is 16 km
below the ellipsoid surface (caused by the curvature of the
Earth).

> ...and Prof. Molodensky preceeded them [Dr. Kumar and Prof. Badekas]
> on another continent by some years if not decades.

That reminds me of something I have wondered about:
Wasn't Helmert much earlier than Burša and Wolf?

Regards,

Mikael Rittri
Carmenta AB
Sweden
www.carmenta.com
________________________________

From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Clifford J Mugnier
Sent: den 12 oktober 2010 16:37
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Using Proj.4 to compress a 10-parameter Molodensky-Badekas datum shift into 7 parameters


The "evaluation point" is the Datum Origin, a rather important item.  In regard to the "unexplained" difference in ellipsoid height results, my guess is the approximations made by the inverse algorithm from Geocentric coordinates.

Personally, I prefer the elegance of incorporating the Datum Origin. 

I have also been told by Dr. Muneendra Kumar that Badekas had nothing to do with Molodensky's original method.  I believe they were in school together at the same time, and Prof. Molodensky preceeded them on another continent by some years if not decades.

Clifford J. Mugnier, C.P., C.M.S.
Chief of Geodesy,
Center for GeoInformatics
Department of Civil Engineering
Patrick F. Taylor Hall 3223A
LOUISIANA STATE UNIVERSITY
Baton Rouge, LA  70803
Voice and Facsimile:  (225) 578-8536 [Academic]
Voice and Facsimile:  (225) 578-4578 [Research]
Cell: (225) 238-8975 [Academic & Research]
Honorary Life Member of the
Louisiana Society of Professional Surveyors
Fellow Emeritus of the ASPRS
Member of the Americas Petroleum Survey Group


________________________________

From: proj-bounces at lists.maptools.org on behalf of Mikael Rittri
Sent: Tue 12-Oct-10 03:45
To: PROJ.4 and general Projections Discussions
Subject: [Proj] Using Proj.4 to compress a 10-parameter Molodensky-Badekas datum shift into 7 parameters

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